The goal of this project is to understand lymphoid organ development and function in ontogeny and inflammation. Directed over expression of lymphotoxin (LT) (either LTa or LTab) results in ectopic lymphoid aggregates with the characteristics of lymphoid organs. These "tertiary lymphoid organs" (TLOs) are also seen in many situations of chronic inflammation in autoimmunity, including the early stages of insulin dependent diabetes mellitus (IDDM). High endothelial venule (HEV) genes are regulated in development, in TLOs, and in acute inflammation. The mature HEV phenotype, characterized by expression of PNAd, LTbR, and a HEV suIfotransferase (HEC-6ST) depends on signaling through the LTbR in normal and HEC6ST- lacZ mice. After immunization, HEVs revert transiently to an immature phenotype and then recover. The recovery is dependent on B cells and LTbR. Lymphangiogenesis also occurs in TLOs and in acute inflammation. At early times after immunization, the occurrence of vessels that are positive for both LV and HEV markers is dependent on LTbR. Results with a novel bimodal dendrimer that visualizes lymphatic vessels by magnetic resonance imaging (MRI) and fluorescence suggest that mice deficient in LT have defective LVs. It is not known how LT contributes to lymphangiogenesis. Two hypotheses will be tested. Hypothesis I. Lymphatic vessels are regulated by LT in inflammation and development;Hypothesis II. Continual signaling through LT is necessary for maintenance and function of lymphoid organs particularly in respect to HEVs and LVs. The specific aims are to: 1. Determine how HEVs and LVs are regulated in acute inflammation by determining if B cells produce the LT ligand that regulates HEVs and LVs, by evaluating the role of macrophages in lymphangiogenesis, and by determining if the double positive vessels represent LVs budding from HEVs;2. Determine how HEVs and LVs are regulated in chronic inflammation by evaluating vessel phenotype, density, and function, the role of macrophages, and LTbR and TNFR signaling;3. Determine if and how LT influences constitutive LVs by evaluating LV function and phenotype 4. Determine if continual LT signaling is necessary to maintain HEVs and LVs in TLOs by treatment with LTbR and TNFR inhibitors and by turning LT on and off with a tetracycline inducible system. Elucidation of HEV and LV regulation will provide strategies for their inhibition in autoimmunity and allow control of insulitis.